Hepatic glucose metabolism is under moment-to-moment control by a variety of signals, both humoral (glucose, insulin, glucagon) and neural (sympathetic and parasympathetic). We are proposing coordinated experimental and computer simulation studies to deduce the mechanisms by which the various factors influence the liver, to determine how they interact, and to evaluate the significance of the complex set of signals to which the liver is exposed during the fluctuations between feeding and fasting. The dynamic effects of hormones on hepatic glucose production will be studied with the in vitro puppy liver, cross-perfused with blood. This preparation is exquisitely sensitive to hormones at concentrations in the physiological range. We shall measure net hepatic glucose balance, rate of production of new glucose, intermediates of the pathway of glycogen metabolism, the rate of cyclic 3'5'AMP release by the liver, and hepatic glycogen content. The dynamic effects on the above variables will be observed in response to various carefully chosen temporal patterns of administration and doses of glucagon, insulin and glucose, either alone or in combination. A computer model of the second messenger system in liver will be developed and, by utilizing the model to simulate the observed dynamics, we shall attempt to delineate the specific sites of action of insulin and glucose in inhibiting glucagon stimulated hepatic glucose output. We have also developed a new preparation which allows us to change the composition of blood perfusing the brain (e.g., by infusing 2-deoxy glucose) without changing the composition of blood perfusing the pancreas or liver. We shall investigate the physiological mechanisms by which neural control of the liver is mediated, and compare the biochemical mechanisms of that control with the mechanisms involved when hormones are infused into the liver directly.